In the past three decades, electricity has risen from 25% to 40% of end-use energy consumption in the United States. With this rising demand for power comes an increasingly critical requirement for highly reliable, high quality power. As power demands continue to grow, older urban electric power systems in particular are being pushed to the limit of performance, requiring new solutions.
Metal conductors, such as copper and aluminum, form the foundation of the world's electric power system, including generators, transmission and distribution systems, transformers, and motors. The discovery of high-temperature superconducting (HTS) compounds in 1986 has led to an effort to develop conductors incorporating HTS compounds for the power industry to replace metal conductors. HTS conductors are one of the most fundamental advances in electric power system technology in more than a century.
HTS conductors can carry over one hundred times more current than conventional metal conductors of the same physical dimension. The superior power density of HTS conductors will enable the development of a new generation of power industry technologies. HTS conductors offer major size, weight, efficiency, and environmental benefits. HTS technologies will drive down costs and increase the capacity and reliability of electric power systems in a variety of ways. For example, an HTS cable is capable of transmitting two to five times more power through existing rights-of-way, thus improving the performance of power grids while reducing their environmental footprint.
However, to date, only short samples of the HTS conductors have been fabricated at high performance levels. For HTS technology to become commercially viable for use in the power generation and distribution industry, it will be necessary to develop equipment for continuous, high-throughput production of HTS conductors.
One way to characterize HTS conductors is by their cost per meter. An alternative way to characterize HTS conductors is by cost per kiloamp-meter. That is, by increasing the current-carrying capacity for a given cost per meter of an HTS conductor, the cost per kiloamp-meter is reduced. This is demonstrated in the critical current density (Jc) of the deposited HTS material multiplied by the cross-sectional area of the material.
For a given critical current and width of HTS material, one way to increase the cross-sectional area is to increase the HTS material thickness. However, under conventional process parameters, it has been demonstrated that with critical current density as a function of thickness, the critical current density drops off as the thickness of a single layer of HTS material increases beyond approximately 1.5 microns and may reach saturation. This is because beyond a film thickness of approximately 1.5 microns, the HTS material becomes very porous, develops voids, and develops increased surface roughness, all of which contribute to inhibiting the flow of current. This results in limiting the critical current in coated conductors to typically 100 A/cm width.
Chemical vapor deposition (CVD) is a process that shows promise for the high throughput necessary to cost-effectively produce HTS conductors. During CVD, HTS material, such as yttrium-barium-copper-oxide (YBa2Cu3O7 or “YBCO”), may be deposited by vapor-phase precursors onto a heated substrate via chemical reactions that occur at the surface of the substrate. Also, CVD and, particularly, metalorganic chemical vapor deposition (MOCVD), is an attractive process for fabrication of HTS coated conductors at a high throughput. High throughput is partly enabled by large deposition areas that are possible with MOCVD.
However, there are several issues that need to be resolved for the uniform deposition of HTS materials over large areas on continuously moving substrates including precursor uniformity, exhaust of precursor waste and substrate temperature uniformity. Injecting a precursor uniformly over a large deposition area of a heated substrate, in particular a moving substrate, can be especially difficult. Further, obtaining sufficiently high conversion of precursor to HTS material formed on is a challenge in particular so as to avoid waste of expensive precursors, and increase deposition rate. Also, depositing HTS material on a moving substrate within the preferred temperature window is challenging.
Thus, there remains a need for a new and improved CVD apparatus that is capable of continuously and uniformly forming a superconducting material and non-superconducting material on the surface of an elongate substrate, especially on continuously moving substrate, to manufacture superconducting conductors.